The present invention in general relates to a method and apparatus for detecting direct currents flowing in the conductors of a transmission line.
In many telephone systems and information transmission systems in general, dc loop currents are generated in the conductors forming the transmission line to serve as a means for signaling or indicating a particular state of use or condition of the system. In telephone systems, these loop currents are sometimes referred to as supervisory signals in that the condition of the signal directs or commands certain responses from the overall system. As an example of such a direct current supervisory signal, some telephone systems employ a dc loop current to detect an "off-hook" condition at the subscriber's end of the transmission line.
The direct current signal is generated by a power source, sometimes symbolized as a battery, at the central office, or other plant, operated by the telephone company. An electro-mechanical switch, located on the subscriber's telephone instrument, is operated by removing the telephone handset from the instrument's cradle causing the "hook switch" to close at the subscriber's station. This permits the direct current to flow in a circuit loop including forward and return conductors provided by the transmission line, sometimes referred to as the "tip" and "ring" conductors, and with the "hook switch" completing the circuit at the subscriber's telephone.
Once a loop current begins to flow, a searching circuit or system at the central office recognizes the "off-hook" condition and transmits a dial tone to the subscriber and establishes a transmission path for use by the subscriber. Similarly, when the call has been completed and the subscriber has returned the hand set to the instrument's cradle, the "hook switch" is opened, terminating the dc loop current. This termination of the loop current may again be sensed by the central office equipment, signaling the fact that the call has been completed by the subscriber, and that the previously established transmission circuit is no longer being used.
Although the foregoing operation illustrates an important use of a supervisory loop current, it is but one application of the use of direct currents within an information signal transmission system. Other uses of the dc loop current include telephone systems in which the loop current is reversed in polarity, to signal a supervisory condition. For example, telephone office equipment based on a "called party answering" system uses a reversal of the direction of the loop current to indicate that the "called" party has answered a ringing signal.
Loop currents are employed as a means of signaling between central offices for controlling trunk lines, and within an office for various signal and control functions.
The loop current may also be employed as a means for detecting whether "loop extenders" and/or "battery boosters" are required in a particular transmission line. In this case, the magnitude of the dc loop current is monitored. If a particular transmission hook up requires an excessively long transmission line, it may be necessary to boost both the information signals and the supervisory signals in order to compensate for losses in the long line. In these cases, it is desirable to provide an accurate indication of the magnitude of the supervisory dc current at any given point in time.
Additionally, the loop current is sometimes used as a means for detecting faults, such as an open or shorted line, in the transmission network. An excessive current might indicate a line short. A lower than expected or zero current would indicate an "open" line.
Traditionally, loop currents have been monitored by using a line relay connected with its coil in series with the transmission line. Sensing relays used in this manner have the advantage of establishing complete electrical isolation between the transmission line itself, and the associated telephone control circuitry that is to be controlled in response to the contacts of the relay. In other words the coil of the relay is connected in the transmission line, however the contacts operated thereby are completely, electrically independent of the transmission line circuit. For this reason and others, sensing relays for detecting dc loop currents have been found satisfactory over the years for use in conventional telephone switching systems.
However, early telephone systems have undergone substantial modification and improvement in terms of efficiency and reliability. In these improved, modern systems, it is not desirable to use switching relays for all current sensing applications because of their relatively high operating power requirements, undesirably large dc resistance, slower operating speed and relatively large size.
in this regard, attempts have been made to find an improved means for use in transmission control systems for detecting loop currents. For example, electro-optic isolators connected in the loop current path of the telephone switching circuit have been proposed, however their use has not been found satisfactory for many applications, such as in transmission line pairs where it is necessary to discriminate between loop currents and common mode currents.